1. Field of the Invention
This invention relates to systems for sensing changes in magnetic fields and methods of fabricating such systems, and more particularly to the use of magnetoresistor circuits in conjunction with moving magnetic parts.
2. Description of the Related Art
Varying magnetic fields have been used in the past as a sensing mechanism for moving parts, such as rotating elements in an automobile. The rotating element causes a magnetic field to periodically vary, and the rate of variation is sensed as an indication of the rotational speed.
A basic form of magnetic field sensing circuit is shown in FIG. 1, while a somewhat more advanced form is shown in FIG. 2. In FIG. 1, a magnetoresistor (MR) 2 is connected in series with a fixed resistor (FR) 4 in a voltage divider circuit. A tap taken between MR 2 and FR 4 is applied to the non-inverting input of an operational amplifier 6, while a voltage reference is applied to the amplifier's inverting input. A voltage V+is impressed across the voltage divider circuit, with FR 4 acting as a load resistor for the amplifier. MR 2 is positioned within a varying magnetic field, such that its resistance varies with the strength of the magnetic field at any given time. For a periodically varying magnetic field, the amplifier output V.sub.o will thus also vary periodically. With the amplifier operated at high gain in a saturated mode, the output V.sub.o will be in the form of a square wave signal for a sinusoidally varying magnetic field, and will have the same frequency as the magnetic field. The rate of magnetic field variation can thus be determined.
In FIG. 2, the same voltage divider circuit consisting of MR 2 and FR 4 is used to establish the noninverting input to amplifier 6. The inverting input, however, is taken from a second voltage divider circuit which is in parallel with the first circuit and consists of series connected FRs 8 and 10. As in the circuit of FIG. 1, MR 2 is positioned within the magnetic field being sensed, while the FRs are separately manufactured and located away from the magnetic field.
The circuit of FIG. 2 is described in a book by International Telephone and Telegraph Company, copyright 1956, Reference Data for Radio Engineers. MR 2 is formed from a magnetoresistive material such as InSb, while the FRs are conventionally manufactured from a non-magnetoresistive material such as composite carbon materials.
A simplified system for measuring the rotational speed of a rotating element with the above type of circuit is shown in FIG. 3. A rotating wheel 12 with a plurality of equally spaced protruding teeth 14 rotates below a magnet 16. A substrate 17 bearing MR 2 is placed in the magnetic field between the magnet and wheel, while the remainder of the circuitry bearing the FRs and amplifier are implemented on a substrate 18 which is outside of the magnetic field away from MR 2.
Rotating wheel 12 is formed from a magnetic material, and thus attracts the field from magnet 16. The magnetic field at MR 2 is strongest when one of the teeth 14 is located directly below magnet 16, thus minimizing the distance between the magnet and wheel. As the wheel rotates, the field tends to bend along with the movement of the tooth, and also traverses a greater distance as the tooth moves away from the magnet. These effects produce a reduction in the magnetic field strength at MR 2, reaching a minimum when the magnet 16 is midway between two teeth 14. The magnetic field strength at MR 2 increases again as the next tooth 14 approaches, reaching a maximum when the next tooth is located directly below the magnet. The field strength at MR 2 thus varies periodically as the magnet continues to rotate, causing the resistance of MR 2 to fluctuate in a similar fashion to produce a periodically varying output from the FR and amplifier circuitry on substrate 18.
A problem with the above approach is that the MR and FRs have different temperature coefficients, causing the circuit operation to vary according to the ambient temperature. Another inaccuracy can result from the fact that the FRs are at a location which is remote from the MR, and can be at a different temperature. In addition, errors stemming from manufacturing tolerances for the resistors can be magnified if the variation of the MR from its nominal value is in a sense opposite to the variation of the FRs from their nominal values. 18. Strict manufacturing tolerances are thus required, which can reduce manufacturing yields. The sensitivity of the described circuits is also not optimum, and additional parts are required because of the separation between the MR and FRs.